1. Field of the Invention
The present invention relates to a method for forming a silica film on a panel and, more particularly, to a method for forming a silica film, which can interrupt electron waves or prevent static electricity, on a face panel for a cathode ray tube.
2. Description of Related Art
Generally, a face panel of a cathode ray tube manufactured through an assembly line goes through a coating process of a nonglare film, an antistatic film, an antireflection film, or/and an electron wave-proof film.
U.S. Pat. No. 5,660,876 to Kojima et al. discloses a method of manufacturing a cathode ray tube with a nonglare multi-layered film comprising at least a visible light absorbing layer containing a black dye, and an antistatic layer containing an inorganic metal compound used as a conductive agent. U.S. Pat. No. 5,523,649 to Tong et al. discloses a video display panel having multi-layered antireflective coating. The U.S. Pat. No. 5,652,477 discloses a display device having a multi-layered antistatic/antireflective coating.
The electron wave-proof film for interrupting the electron waves is usually made of a single-layered structure having a silica layer or a dual-layered structure having antistatic and silica layers.
The antistatic layer comprises a transparent conductive coating layer made of a material selected from the group consisting of an ITO and ATO or comprises a thin metal layer made of Ag, Ag/Pd. The silica layer is made through a sol-gel method.
As a well-known method for forming a silica film, water for a hydrolysis reaction is added to silicon alkoxide. Alcohol is used as common solvent of the silicon alkoxide and the water. In addition, acid is used as a catalyst to derive hydrolysis and polycondensation so as to form a thin film.
Describing more in detail, in the sol-gel method for forming a silica thin film, the silica film is made through a sol composition, coating, drying, and heating treatment processes. That is, composition of the silica sol is realized by adding a predetermined amount of water to a precursor such as, for example, a tetraethyl-o-silicate or a tetramethyl-o-silicate so as to derive hydrolysis reaction. At this point, since the silicon alkoxide is hydrophobic, a common solvent such as ethanol, methanol, or butanol is further added. After this, catalyst is further added to provide a sol particle structure that is suitable for being formed into thin film by a spin coating process or a dip-coating process and for accelerating hydrolysis reaction and polycondensation reaction. Generally, the catalyst is selected from the group consisting of organic or inorganic acids such as hydrochloric acid, nitric acid, acetic acid, and phosphoric acid, all of which have high acidity, and yet are dangerous agents that pose health risks.
In addition, as the thin film reacts to an acid-ionic group, if the thin film is a dual-layered structure, the thin film changes in physical property.
In the above description, the structure suitable for forming a thin film refers to the connecting structure of the silicate having a three-dimensional network structure or a linear structure having a siloxane bone, on one side of which a silanol group is coupled.
The kind of alkoxide, the amount of water, and the kind of alcohol have affect on the form of the resultant sol. However, when the amount of the water is determined, the form of the resultant sol is affected by the kind and amount of acid added as the catalyst.
Generally, in the binary system alkoxide reaction, a hetero condensation reaction between hydrolysises of each alkoxide occurs prior to the alkoxide reaction.
That is, the alkoxides M1 and M2 produce hydroxides through hydrolysis reactions as follows: EQU M1(OR)n+nH.sub.2 O.fwdarw.M1(OH)x(OR)n-x+xROH EQU M2(OR)n+nH.sub.2 O.fwdarw.M2(OH)x(OR)n-x +xROH
Each of the hydroxides produced as in the above converts oxygen bridge into a two-component condensation product of M1-O-M2 by water condensation between the hydroxides rather than by its homo condensation, thereby being accelerated in its reaction. This can be shown as follows: EQU M1(OH)(OR)+M2(HO)(OR).fwdarw.(OR)M1-O-M2(OR)+H.sub.2 O
However, in case of the silica-titania two-component system, titanium alkoxide is used as a catalyst which can accelerate polycondensation reaction of the silicate. Chelate derivative can be also used as the catalyst that can adjust haydrolysis reaction speed of the titanium alkoxide.
Therefore, when the colloidal solution is coated on a face panel of a cathode ray tube, this becomes the silica film. That is, after the face panel is applied with the colloidal solution, it is calcinated at a temperature of about 300.degree. C. so that the organization of the film is fined and the attachment thereof can be stabilized.
However, since the heat treatment process is locally performed in the cathode ray tube, differences in thermal expansion between the panel, explosion proof band, and a funnel occur. As a result, if severe, the cathode ray tube can crack or the coated film is disproportioned.